Beam radiation sources such as lasers are very useful in producing images by utilizing different types of scanning over the surface to be imaged. Although some types of energy, (E.G. electron beams) may be deflected scanning by electric or magnetic fields, laser beam scanning is typically accomplished by electromechanical means, such as sets of mirrors driven by servomotors, which in turn are controlled by suitable computer devices. These electromechanical means operate relatively slowly as compared with their electrically or magnetically controlled counterparts.
Three dimensional imaging has recently become of great importance. The problems encountered in three dimensional imaging are considerably more complex than those presented in two dimensional imaging, due to the additional necessity of accurately controlling the exposed depth, and due to the multilayered structure of three dimensional models.
Many systems for production of three-dimensional modeling by photocuring have been proposed. European patent application (Publication No. 250,121 filed by Scitex Corporation, Ltd. on June 6, 1987), and incorporated herein by reference, provides a good summary of documents pertinent to this art area, including various approaches attributed to Hull, Kodama, and Herbert. Additional background is described in U.S. Pat. No. 4,752,498 issued to Fudim on June 21, 1988, which is also incorporated herein by reference.
These approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photocurable composition with a laser beam according to a desired pattern and building a three-dimensional model layer by layer. They utilize raster scanning, which however has many disadvantages, such as necessity to scan the whole field even if the object to be produced is only a very small part of the total volume, considerably increased amount of data to be stored in most cases, overall more difficult manipulation of the stored data, and the necessity to convert CAD-based vector data to raster data.
An alternative way of scanning would be vector scanning, where only the areas corresponding to the shape of the rigid object have to be scanned, the amount of data to be stored is smaller, the data can be manipulated more easily, and "more than 90% of the CAD based machines generate and utilize vector data" (Lasers & Optronics, Jan. 1989, Vol. 8, No. 1, pg. 56). Vector scanning, despite its advantages, introduces problems related to the inertia of the optical members, such as mirrors, of the available deflection systems for the currently most convenient radiation sources, such as lasers. Since these systems are electromechanical in nature, there is a period of acceleration involved in reaching any desired beam velocity. This unavoidable non-uniformity in velocity results in unacceptable thickness variations in the exposed photocurable composition. Especially in the case of portions of layers having no immediate previous levels of exposure at the high intensity it becomes necessary to use high beam velocities, and therefore, longer acceleration times, which in turn result in thickness non-uniformity in the exposed composition. Our co-pending application IM-0178, incorporated herein by reference, describes an improved method and apparatus to utilize more effectively vector scanning and avoid its previously experienced disadvantages. However, even in this case, the mechanisms employed are rather complicated and expensive.
It is therefore an object of the present invention to provide a simple and inexpensive method and apparatus for direct writing on a surface with radiation in the form of a beam, and particularly in the form of a laser beam by an improved way of scanning, where the exposed portions of the surface receive a controlled level of exposure, which level can be caused to remain substantially constant.
It is a further object of the present invention to provide a simple and inexpensive method and apparatus for direct laser writing onto an unexposed photocurable composition by the aforementioned improved way of scanning, and forming sequential three-dimensional model layers of precisely-controlled depth and resolution.